A connecting rod is generally known as a rigid member that provides the mechanical linkage between a piston of an engine, particularly a reciprocating engine such as an internal combustion engine, and a crank or crankshaft. The connecting rod functions as a lever arm by pushing and pulling the piston into and out of the cylinder, and converts the linear up-and-down movement of the piston into rotation of the crankshaft. This motion is then passed on to a series of devices capable of providing power to the machine, e.g., vehicle, in which the engine is equipped.
A common measure of engine power, which is dependent upon the connecting rod, is compression ratio, defined as the ratio between the swept volume of a cylinder with the piston at bottom dead center (BDC) and the swept volume of the cylinder with the piston at top dead center (TDC). Put more simply, compression ratio can refer to the ratio of maximum volume to minimum volume in the cylinder. When the connecting rod has a fixed length, the engine will have a fixed displacement and compression ratio, as the maximum and minimum volume in the cylinder are constant.
A fixed compression ratio, problematically, can result in missed performance optimization. For instance, under low engine loads, such as idling, a higher compression ratio can yield improved fuel economy. Meanwhile, under high engine loads, such as a large power request from the driver, a lower compression ratio, combined with increased boost, can yield improved power.